The present invention relates to an image processing apparatus adapted for use in an image forming apparatus, which is, for example, a color copying machine for forming color images.
A conventional image forming apparatus (color copying machine) forms an image, using four colors, namely, C (cyan), M (magenta), Y (yellow) and K (black). For this image formation, the color copying machine employs an image processing apparatus that converts an input image, which is comprised of image information read by a color scanner, into four-color (C, M, Y and K) image signals. The input image includes characters, line drawings or photographs. To cause the characters and line drawings to look sharp and clear, it is desired that a black (achromatic) image be formed in K only. On the other hand, a halftone image has to be printed as smoothly as possible. For this reason, it is desired the halftone image be printed in mixed colors of CMYK whether the original color of that image is black (achromatic) or other colors (chromatic). To meet this desire, the image processing apparatus of the color copying machine checks each of the pixels of an input image and determines whether it corresponds to a region of characters and line drawings or a region of halftone images.
For example, the image processing apparatus determines that the pixels of a character or a line drawing are achromatic when they are look similar to black; otherwise, it determines that the pixels are chromatic. Based on this determination by the image processing apparatus, the color copying machine executes image formation in such a manner that a black character or line drawing is developed in black (K) only, and that a halftone image and a color character or line drawing is developed in color (CMYK).
In the case of a color image, the edge of a character or a line drawing may undergo a color variation for each color component. For example, the color scanner of a color copying machine reads the density of a pixel for each color component, and a significant density difference between the color components may be detected on the edge of a black character.
If such a color variation occurs on the edge of a black character or the like, the conventional image processing apparatus may undesirably determine that the color variation portion is chromatic. As a result, the color copying machine executes image formation in such a manner that an image portion that should be printed in black is printed in color (CMYK).